Fixed default StorageClass from 'regular' to 'standard' per API spec
Fix and test multipart storage class

2f90f973076b7b9b5bbec62bc1f90ac258738236 fixes newly written multipart
uploads to use the proper storage class, but this just ignores the value
alltogether and uses `STANDARD` no matter what. It also adds a test in
the python test suite.
Merge branch 'bugfix/multipart-storage-class' into develop